The present invention relates to control of various variables in emissions in combustion. More particularly the invention relates to use of carbon monoxide emissions in pulverized coal fired boilers to achieve low NOx production and high efficiency burning.
The use of pulverized coal fired boilers has long been a source of energy. As concerns for fuel burning efficiency and emissions has become more important, as part of an increasing world-wide concern for conserving energy and protecting the environment, various methods have been proposed for reducing undesirable emissions from combustion plants.
Reduction of the production of NOx is particularly desirable since this emission product is recognized as one of the chief sources of acid rain, in addition to SO2, of course, and is a major problem in some areas of the world where industrial emissions from burning hydrocarbon fuels react with gases in the atmosphere to produce acidic compounds that fall as rain.
Woolbert U.S. Pat. No. 4,852,384 initiates a calibration sequence for a combined oxygen and combustibles analyzer, using an automatic periodic calibration system with a signal sensing and safety alarm system. Both oxygen and the fuel are analyzed and controlled. The system is designed to replace manual testing where an operator introduces a test gas into the system.
Dykema U.S. Patent No. U.S. Pat. No. 5,215,455 employs a plurality of combustion zones and stages while regulating temperatures of combustion. At least two stages are used in which the first combustion zone is fuel-rich to convert chemically bound nitrogen to molecular nitrogen. The second zone includes two combustion stages that are said to avoid production of NOx because of a cooling step substantially lowering the final combustion temperatures.
Koppang U.S. Pat. No. 5,759,022 uses a secondary burn zone downstream from the primary burn zone to reduce production of NOx, also in a fuel-rich mixture. This patent relates to liquid and gas hydrocarbon fuels, and depends upon intermixing these fuels with oxygen during the process. Koppang is said to be an improvement on Quirk et al. U.S. Pat. No. 5,849,059, which patent reacts waste gasses in a glass furnace. This reference uses a secondary combustion by adding air to exhaust gases as they leave a regenerator to combust and remove combustible material in the waste gas before exiting to atmosphere.
Ashworth U.S. Pat. No. 6,085,674 discloses three stages of oxidation in which gas and preheated air are introduced in stages. NOx production is reduced by first partially combusting the fuel in the presence of heated combustion air, then removing molten slag in the second stage and causing further combustion. The flue gas then is combusted in a third stage to complete combustion of the fuel NOx is said to be reduced by controlling the stoichiometric rations at each stage of combustion.
Finally, Miyagaki U.S. Pat. No. 4,622,922 uses images from cameras and fiber optics to control combustion. The amount of NOx and unburned coal in the ash are measured and a trial and error process is used in trial operations to attempt to achieve stability of combustion and meet some standard of emission output.
None of the prior art is able to effectively control the production of NOx and optimize the efficiency of the combustion when optimization of combustion air is not constant but is uncertain. Variations in CO production during combustion have prevented full optimization of NOx, production, and have not allowed comparison of efficiency and emission of undesirable components such as NOx.
It would be of great advantage in the art if a method could be produced that would utilize information relating to combustion uncertainty and its relationship to formation of CO emissions in pulverized coal fired boilers.
It would be another great advance in the art if analysis of the emissions could be done in a region of the system where CO emissions occur with prescribed probability.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention provides method of controlling combustion of fuel in a boiler having an adjustable air-to-fuel ratio.
The CO production at any given time in a boiler will be very highly varied. There is no linear relationship to use to pick any given point for use in controlling the air to fuel ration because, of course, at any given time a median, since the distribution is not symmetrical will have 50% of the CO production above that point and 50% below that same point. For that reason, the present invention includes the measurement of CO production over time to provide a CO distribution range during that period of time for the combustion.
Similarly, the NOx production will give the same wide deviation from any average measurement. Accordingly, the measurement of NOx, production is done over time to provide measuring the NOx, production to provide a NOx, distribution range during said period of time.
In both cases, it is necessary to estimate the distribution of CO and NOx, emissions, then select a set point, such as 90% or 95% of the total distribution, by way of example, so that during operation, the emissions will remain below the maximum permitted level with the probability given by said set point. The operator is then able to adjust the air-to-fuel ratio to cause combustion of the fuel within the CO distribution range and the NOx distribution range to thereby cause the actual emission for said CO and NOx distributions to average less than the maximum permitted.